Thermocouples have been used for many years to make accurate temperature measurements. When in use, a thermocouple generates a temperature-related, thermoelectric voltage that can be applied to an instrument or other device to produce useful information. The information may, for example, be a display of the temperature or control signals that can be used to control other equipment. The thermocouple may be connected to an instrument directly, or by thermocouple extension wires. The thermo-electric voltage formed between the junction and the open end of the thermocouple is applied to the terminals of the instrument. The temperature-related voltage at the instrument terminals is combined with the temperature of the terminals to determine the temperature at the thermocouple junction.
The temperature of the instrument terminals may be sensed by a temperature sensor that is typically, but not necessarily, located near the terminals. To ensure accurate temperature readings by the instrument, it is important to minimize any temperature differences between the instrument terminals as well as between the terminals and the temperature sensor. That is, since the thermoelectric voltage is (at least in part) dependant on the temperature of the instrument terminals, a temperature difference between the terminals, or between the terminals and the terminal temperature sensor, may cause the instrument to make erroneous temperature readings. In order to alleviate this problem, thermocouple wires are usually connected to the instrument via an isothermal termination block that maintains the terminations at the instrument and a companion terminal temperature sensor at substantially the same temperature, i.e., isothermal.
Prior art isothermal termination blocks typically consist of a printed circuit board with input terminals, a terminal temperature sensor and a thermal conductor. The thermal conductor is usually made of a block of copper or aluminum or other material having good thermal conductivity. Typically, the thermal conductor is screwed or bolted to an exterior surface of the circuit board. In some cases the thermal conductor may be soldered to the exterior surface of the circuit board or affixed by a thermally conductive adhesive. In any event, the thermal conductor is positioned on the circuit board near the input terminals and the temperature sensor so that it thermally couples the terminals and the sensor, thereby maintaining them at the same, or substantially the same, temperature, which as noted above, is important for ensuring accurate temperature measurements.
Typically, thermal conductors are electrically conductive in addition to being thermally conductive and, therefore, must be isolated from electrical signal carrying portions of the circuit board, such as the input terminals and the local temperature sensor. Because temperatures often must be measured in the presence of AC and DC line voltages, electrifying the thermal conductor could cause damage to other equipment as well as threaten the safety of operating personnel. The thermal conductor is isolated from the terminals on the circuit board by maintaining minimum creepage and clearance distances therebetween so as to prevent voltages at the terminals from creating a conductive path or arcing to the thermal conductor. These creepage and clearance distances are established by accepted industry standards, such as ANSI/ISA-S82.01-1988, IEC 1010, and CSA C22.2, No. 231.
The clearances established by these standards take into account such variables as rated line voltages on the terminals and the type of insulator between the terminals, sensor and the thermal conductor. In the case of the prior art isothermal blocks described above (i.e., where the thermal block is mounted on the external surface of the circuit board) air and the circuit board surface form the insulators. With air as an insulator, the safety standards require a relatively large distance, or clearance and with circuit board surfaces as an insulator, the safety standards require a relatively large creepage distance between the terminals, sensor and the thermal conductor. Unfortunately, the clearance and creepage safety spacings reduce the thermal coupling between the input terminals, temperature sensor and thermal conductor, especially when air is the insulator because the required minimum spacing is so great. Thus, one way to improve the thermal coupling is to place the thermal conductor close to the terminals and the sensor. Unfortunately, safety standards limit how closely they can be spaced apart and therefore limit, to a significant degree, the thermal coupling.
Another way to improve the thermal coupling between the input terminals and the sensor is to increase the size of the thermal conductor (while still maintaining adequate safety spacings). Prior art attempts to accomplish this include increasing the thickness of the thermal conductor. In fact, in many instances the prior art thermal conductor is substantially thicker than the printed circuit board to which it is attached. Unfortunately, increasing the thickness of the thermal conductor creates an isothermal block that is both massive and expensive to manufacture.
Accordingly, there is a need for an isothermal block having a small, light weight, thermal conductor that is inexpensive to manufacture and provides improved thermal coupling. The present inventing is directed to an isothermal block having a multi-layer thermal conductor embedded in a printed circuit board designed to achieve these results.